If you do miss a package, either because you pass through town when the P.O. is closed or because it's slower than expected, it's still easy to get it. Take a piece of paper (or index card), write something like "Please forward all general delivery mail for Connie 808 to X," where X is your next best guess (2-4 days up the road, similar criteria as before). Sign the card, date it, drop it in the "local delivery" box in the post office, and ride off into the sunset.I have never done that, but do have another method that has worked fine. You can stop and any post office, not just the one the package is at, and request the forwarding. You can even do it multiple times for the same package.
On one trip we were given a large amount of dried and freeze dried food and shipped it ahead to ourselves. We kept getting to where it was shipped before we needed it and forwarded it several times. They didn't charge for the forwarding. I would not ship food ahead via general delivery again unless maybe it was a mountain bike trip that was more like backpacking than bike touring, but for getting stuff from home it is a great option.
General delivery works way better than shipping to a store, bike shop, or warmshowers.org host. We first tried having things shipped to a business along the way (with their prior approval), but when the package arrived late we either needed to wait around, abandon the package, or have them forward it to us. Fortunately they reshipped it ahead for us for only the cost of the postage.